In recent years, to meet a requirement of energy saving, various types of so-called hybrid working machines have been developed that have a prime motor and a power generator mounted thereon as a power system of a working machine such as a hydraulic excavator, or the like. This hybrid working machine is capable of mounting thereon a motor-operated or a motor-operated hydraulic actuator driving device having, for example, the following structure.
The motor-operated actuator driving device is operable to drive an actuator directly by an electric motor, or via a reduction mechanism, wherein a operation direction of the actuator is controlled by a rotation direction of the electric motor, and a speed of operation of the actuator is controlled by a revolving speed of the electric motor.
The motor-operated hydraulic actuator driving device is operable to drive a hydraulic pump by rotating the electric motor, which hydraulic pump drives the actuator. Where, a discharge direction of the hydraulic pump is controlled by the rotation direction of the electric motor, and an amount of discharge from the pump is controlled by the revolving speed thereof, so that the operation direction and speed of the actuator is controlled (as disclosed in Japanese Patent Unexamined Publication No. 2001-2371).
In an AC servo motor, an operable area in the case of continuous use or repeated use, depending on working time of the electric motor, is defined by a relationship between a revolving speed of the electric motor and torque. thereof. Otherwise, available working time of the servo motor is defined according to torque of the electric motor or load ratio thereof. This is based on limits due to a raise in temperature of the electric motor and a controller. Thus, when a prescribed work is intended to be carried out by the motor-operated actuator device, the electric motor should have a larger size, and its power supply capacity be increased, leading to higher manufacturing costs.
Then, downsizing of the electric motor or power supply capacity results in low manufacturing costs, but restricts available continuous working time. Since, especially in a hydraulic excavator, a mobile crane, or the like, a space to set therein devices is strictly limited, devices such as an electric motor, a controller, a power supply, or the like may be preferably as small as possible. However, in cases where the devices have usable conditions thereof strictly restricted, they are difficult to handle. Accordingly, the problem encountered with the related art is that both requirements concerning the size of device and the length of working time thereof are not compatible with each other at the same time. On the other hand, the motor-operated hydraulic actuator driving device is supposed to have the same problems.